Number of surjections from an n-element set onto a three-element set, with n >= 3. - Mohamed Bouhamida (bhmd95(AT)yahoo.fr), Dec 15 2007

Let P(A) be the power set of an n-element set A and R be a relation on P(A) such that for all x, y of P(A), xRy if either 0) x is not a subset of y and y is not a subset of x and x and y are disjoint, or 1) x is a proper subset of y or y is a proper subset of x and x and y are intersecting. Then a(n+1) = |R|. - Ross La Haye, Mar 19 2009